1. Field of the Invention
The present invention relates to a focus detection apparatus which is mounted on an optical instruments, such as a microscope and an optical measuring machine, to detect a focus of a sample.
2. Description of the Related Art
Conventionally, there is known a microscope provided with a focus detection apparatus which automatically adjusts a focus on a sample to obtain the proper focus. Such a focus detection apparatus is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 60-042725. FIG. 12 schematically shows a configuration of the disclosed focus detection apparatus 1. FIGS. 13A, 13B and 13C are views showing a positional relationship between the sample, an objective lens, and a photodetector, and an image forming state when collecting light reflected from the sample onto a light receiving surface.
Particularly, FIG. 13A shows a light collecting state of the reflected light in the case where the sample is located nearer than a focal position of the objective lens (near focus state). FIG. 13B shows a light collecting state of the reflected light in the case where the sample is located at the focal position of the objective lens (focused state). FIG. 13C shows a light collecting state of the reflected light in the case where the sample is located farther than the focal position of the objective lens (far focus state).
In the disclosed focus detection apparatus 1, as shown in FIGS. 13A to 13C, a position and a size of an image 17 focused on a photodetector 11 are varied according to a change in relative distance between a sample 8 and an objective lens 6, which changes amplitudes of output signals output from light receiving surfaces 11a and 11b. In the focus detection apparatus 1, a signal processing system (not shown) which receives the output signals can determine a focus direction and focused and defocused states.
However, a pupil diameter of an objective lens depends on a type and magnification of the objective lens. For this reason, a sufficient luminous flux emitted from a light source is not always incident to the pupil diameter of the objective lens. That is, the sufficient light reflected from the sample necessary to detect the focus is not always received by a light receiving surface.
For example, when a diameter of the luminous flux emitted from the light source unit is smaller than the pupil diameter of the objective lens, an object-side numerical aperture (NA) of the emitted luminous flux becomes smaller as compared with the object-side numerical aperture of the objective lens. As a result, a movement amount of a representative position of a spot image is decreased on the light receiving surface with respect to a relative distance movement between the objective lens and the sample. That is, detection accuracy drops off because the focal depth is increased.
When the diameter of the luminous flux emitted from the light source unit is larger than the pupil diameter of the objective lens, a part of the luminous flux is cut off by the pupil of the objective lens. Therefore, a light quantity which can effectively be utilized is decreased to lower the detection accuracy.
The problem created by the difference in pupil diameter between the objective lenses will be described with reference to FIGS. 14A, 14B and 14C. FIGS. 14A, 14B and 14C show the difference in pupil diameter between the objective lenses based on the case where the pupil diameter of the objective lens becomes maximum. These figures show the difference in size of the light-receiving-side luminous flux diameter which depends on the size of the pupil diameter.
FIG. 14A is a view showing a state in which, for example, in an objective lens 6 having the largest pupil diameter 18, a luminous flux S1 having an optimum diameter for sufficiently satisfying the largest pupil diameter 18 is transmitted through the objective lens 6 after the luminous flux S1 is transmitted through the objective lens 6 and reflected from the sample 8. FIG. 14B is a view showing a state in which the luminous flux S1 is transmitted through, for example, an objective lens 6 having the smallest pupil diameter 19 after the luminous flux S1 is transmitted through the objective lens 6 and reflected from the sample 8. As shown in FIG. 14B, a part of the luminous flux S1 is cut off, the remaining part of luminous flux S1 is transmitted through the objective lens 6 and transmitted through the objective lens 6 after reflected from the sample 8.
In the state in which the luminous flux S1 is transmitted through the objective lens 6 after the luminous flux S1 is transmitted through the objective lens 6 and reflected from the sample 8, the light quantity of the luminous flux S1 transmitted through the objective lens 6 having the smallest pupil diameter 19 is smaller than that of the luminous flux S1 transmitted through the objective lens 6 having the largest pupil diameter 18. Therefore, lack of the light quantity is generated in the light receiving surfaces 11a and 11b. Additionally, light quantity loss of the laser beam focused on the light receiving surfaces 11a and 11b of the photodetector 11 is easily occurs.
For example, as shown in FIG. 14C, it is assumed that the largest pupil diameter 18 is set at 9 mm while the smallest pupil diameter 19 is set at 3 mm. When the light quantity ratio is obtained between the two, the light quantity ratio becomes 9:1 because the light quantity ratio is equal to an area ratio in the pupil surface. Thus, when the light quantity ratio is large, the quantity of light received by the light receiving surfaces 11a and 11b becomes insufficient. The quantity of effectively usable light received by the light receiving surfaces 11a and 11b becomes further insufficient under such additionally adverse conditions that the objective lens 6 having the smallest pupil diameter 19 is used to perform the focus detection operation to the sample 8 having low reflectivity.
The focus detection apparatus 1 includes an integrating circuit which processes a signal output from the photodetector 11. Conventionally, a time constant and an integration time of the integrating circuit are switched in the focus detection apparatus 1. The focus detection apparatus 1 performs an electrical amplification process to the weak signal output from the photodetector 11 by the switching between the time constant and the integration time. In addition, the focus detection apparatus 1 generates a focus error signal through the amplification process. However, there is a limitation in the amplification process. When the amplification process exceed the limit, a noise component such as temperature change which is varied in time series is added to the focus error signal. As a result, the signal-to-noise ratio decreases in the focus error signal to lower the focus accuracy.
The above problem has been serious in optical instruments such as a microscope in which a magnification of an objective lens is frequently switched between the low magnification and the high magnification.
Therefore, for example, Jpn. Pat. Appln. KOKAI Publication No. 62-143010 discloses a focus detection apparatus including a light source and light receiving means. The light source projects light onto an object through a neighbor of a pupil of an objective lens along an optical path which corresponds to one of optical paths on an optical axis in a focused state of the objective lens. The light receiving means receives the light reflected from the object through the objective lens. In the focus detection apparatus which makes focus determination by detecting displacement of a position of the light incident to the light receiving means, optical path switching means for switching the light from the light source so as to correspond to another optical path on the optical axis is provided between the light source and the objective lens, and a luminous flux is projected so as to coincide with the pupil diameter of the objective lens by moving the optical path switching means in an optical axis direction.
Jpn. Pat. Appln. KOKAI Publication No. 05-045573 discloses a similar focus detection apparatus.
In a configuration of the focus detection apparatus, measurement luminous fluxes emitted from at least two measurement light sources are received by light receiving means arranged in a peripheral portion of a pupil of an objective lens, and focus determination is made by detecting displacement of a position of the light incident to the light receiving means. Because at least the two measurement light sources are provided, any one of the measurement light sources is selectively used according to the magnification of the objective lens.